1. Field of the Invention
The present invention relates to a cell balance control device which equalizes a cell voltage of a battery cell.
2. Description of the Related Art
It is well known that vehicles such as electric cars and hybrid cars are mounted with a motor and a high-pressure, high-capacity battery. The motor acts as a power source. The high-pressure, high-capacity battery supplies electric power to this motor. This battery is structured by connecting a plurality of battery cells in series. The battery cell includes a lithium-ion battery, a hydrogen nickel battery, and the like. Conventionally, in order to maintain the functionality of a battery, a cell balance control is performed by monitoring a cell voltage of each battery cell and equalizing each cell voltage.
Japanese Unexamined Patent Application, First Publication No. H8-19188 (hereinafter referred to as “Patent Document 1”) discloses a technology which connects a direct circuit of a resistance and a switching element with each cell in series as a bypass circuit. The lowest cell voltage is compared with another cell voltage. If this difference in voltage exceeds a first predetermined value, the switching element of the cell is turned on. In this way, a conduction is made in the bypass circuit (i.e., the cell is discharged). When the difference in voltage becomes less than or equal to a second predetermined value, which is smaller than the first predetermined value, the switching element is turned off. In this way, the bypass circuit is shut off. Thus, each cell voltage is equalized.
Japanese Unexamined Patent Application, First Publication No. 2008-21589 (hereinafter referred to as “Patent Document 2”) discloses a technology which is similar to the technology disclosed in Patent Document 1. According to Patent Document 2, a bypass circuit is connected in parallel to each cell. A predetermined capacity adjusting current value is altered based on a capacity adjusting ability enhancement request degree which is set according to a condition that the battery is in (for example, a condition in which a cell voltage is distributed, a proportion of a time during which a battery is used, and a capacity degradation coefficient). A duty of the switching element is controlled so that the bypass circuit is supplied with a discharge current corresponding to the predetermined capacity adjusting current value. In this way, each cell voltage is equalized.
As described above, according to conventional technology, a cell balance was controlled based on, for example, a difference between cell voltages, a condition in which a cell voltage is distributed, a proportion of a time during which a battery is used, and a capacity degradation coefficient. Meanwhile, when a cell balance is controlled, a discharge current runs through a bypass circuit. As a result, the bypass circuit heats up. This leads to an increase in the temperature of a circuit substrate. A bypass circuit, a cell voltage detection circuit, a control circuit (control IC), and the like are mounted on the circuit substrate. In addition to the temperature of the circuit substrate increasing, the circuit element might be destroyed, and malfunctions might occur.
Taking into consideration that the temperature of the circuit substrate might increase, a cell balance control is not performed in conventional technology. A switching element of a plurality of cells might be turned on simultaneously. When such an instance occurs frequently, the temperature of the circuit substrate increases. Further, the circuit element might be destroyed and/or a malfunction might occur. In this way, it might become difficult to properly perform a cell balance control.
The present invention is made according to these considerations. An object of the present invention is to provide a cell balance control device which can prevent malfunctions and destruction of the circuit element due to an increase in the temperature of the substrate, thereby maintaining an appropriate cell balance control.